1. Field of the Invention
The present invention relates to an optical scanning device, a scanning optical system, a scanning imaging optical component, an optical scanning method, a ghost image preventing method and an image forming apparatus.
2. Description of the Related Art
An optical scanning device which deflects a beam from a light source by a light deflector, condenses the deflected beam toward a surface to be scanned by a scanning imaging optical system, forms a beam spot on the surface to be scanned and performs optical scanning of the surface to be scanned, is widely well-known in connection with a digital copier, an optical printer, a facsimile machine, and so fourth.
In such an optical scanning device, ideally, the surface to be scanned is scanned only by a proper scanning light. However, various optical components disposed from the light source to the surface to be scanned reflect light in some degree, and light thus reflected by these optical components and reaching the surface to be scanned acts as stray light against the proper scanning light.
Some stray light which generates a ghost image causing degradation in image quality of an image formed through the proper scanning light and is harmful is called xe2x80x98ghost lightxe2x80x99. Generally, a ghost image generated due to ghost light has a form in which a black stripe overlaps a proper image.
Respective optical components disposed on a light path extending from the light source to the surface to be scanned may generate ghost light.
In a first part of the light path extending from the light source to the light deflector, the light path of the beam is fixed. Accordingly, it is relatively easy to prevent ghost light from being generated, by adjusting the dispositions of the optical components properly.
However, in the second part of the light path extending from the light deflector to the surface to be scanned, the light path dynamically changes as a result the beam being deflected. Accordingly, ghost light may be easily generated from unexpected parts.
As the light deflector, one which reflects the beam from the light source by a deflection reflective surface which is rotated or swung is generally used. Such a light deflector is contained in a housing in order to avoid adherence of dust to a driving part or the deflection reflective surface of the light deflector, or to avoid leakage of machine noise or air flow noise therefrom, and, via a transparent parallel plate which covers a window formed in the housing, the beam is incident on the light deflector and exits from the housing, in many cases.
In such a case, light reflected by the transparent parallel plate covering the window of the housing may become ghost light. In order to prevent the ghost light in this case from adversely affecting proper image formation, the parallel plate is inclined so that the reflected light strays in a sub-scanning direction. However, thus-straying light may become secondary ghost light as a result of being reflected by a part of the scanning imaging optical system.
An object of the present invention is mainly to effectively reduce or prevent generation of ghost image due to reflection by a non-effective surface of the scanning imaging optical component such as a lens disposed between the light deflector and surface to be scanned in the optical scanning device.
A scanning imaging optical component used in an optical scanning device which deflects a beam from a light source by a light deflector, condenses the deflected beam toward a surface to be scanned by a scanning imaging optical system, forms a beam spot on the surface to be scanned, and performs optical scanning of the surface to be scanned,
the optical component forming at least a part of the scanning imaging optical system; and
at least a part of a surface of said optical component other than an effective optical surface is made to be a roughened surface, and roughness thereof is such that:
Rmax greater than 0.5 (xcexcm)
The scanning imaging optical component may comprise a lens, may comprise an imaging mirror having an imaging function, or may comprise a mirror having no imaging function.
The effective optical surface is lens surfaces or parts of the lens surfaces which participate formation of a beam spot, when the scanning imaging optical component is a lens. The effective optical surface is a mirror surface or a part of the mirror surface which participates formation of a beam spot, when the scanning imaging optical component is a mirror.
As mentioned above, the scanning imaging optical component is an optical system which forms at least a part of the scanning imaging optical system. Accordingly, the scanning imaging optical system may include only the scanning imaging optical component, for example. When the scanning imaging optical system includes only a single fxcex8 lens, this fxcex8 lens is the scanning imaging optical component, and at least a partial surface of the effective optical surfaces thereof is made to be the roughened surface.
Alternatively, when the scanning imaging optical system includes a plurality of lenses, each lens may be the scanning imaging optical component which has the roughened surface other than the effective optical surfaces thereof; or only one or some of the plurality of lenses may be the scanning imaging optical component which has the roughened surface other than the effective optical surfaces thereof and the other lenses may have no roughened surface.
The scanning imaging optical system may include a mirror having no imaging function (plane mirror for bending a light path) as the scanning imaging optical component. In this case, at least a part of the mirror surface thereof other than the effective optical surface may be made to be the roughened surface.
When a stray light other than a proper scanning beam is incident on the roughened surface of the scanning imaging optical component, the stray light reflected thereby is dispersed by the roughened surface. Accordingly, even it reaches the surface to be scanned, concentration of light intensity thereof is low. Therefore, even if it forms a latent image, the density thereof is low, and, when the latent image is visualized, no black stripe which problematically degrades a proper image results therefrom.
When the roughness Rmax of the roughened surface is not larger than 0.5 xcexcm, dispersion of the stray light reflected thereby is not sufficient, and, thereby, this reflected light may still act as ghost light when reaching the surface to be scanned, and form a ghost image as a somewhat wide gray stripe.
The roughness of the roughened surface is preferably large in the view point of preventing generation of ghost image. However, when the scanning imaging optical component is formed through molding of plastic material, too large surface roughness Rmax thereof may result in difficulty of sliding of the product when it is separated from the mold. Therefore, in such a case, the roughness Rmax of the roughened surface of the scanning imaging optical component may be preferably such that Rmax less than 10 (xcexcm).
When the scanning imaging optical component is formed of a glass, there is no such a problem for removing the product from a mold. Accordingly, the roughness Rmax may be on the order of 100 xcexcm.
The light deflector of the optical scanning device according to the present invention may be a rotary mono-surface mirror, a rotary bi-surface mirror, a rotary polygon mirror, a galvano mirror, or the like, for example.
In a scanning optical system according to the present invention,
the light deflector is contained in a housing;
a window is provided in said housing for causing the beam from the light source to be incident on the deflection reflective surface, and, also, causing the deflected beam reflected by the deflection reflective surface to exit from the housing;
a transparent parallel plate is provided for covering the window therewith; and
the transparent parallel plate is inclined with respect to a direction perpendicular to a deflection scanning plane.
As the transparent parallel plate is thus inclined, a reflected beam which strays in the sub-scanning direction with respect to a proper scanning beam is generated as a stray light. However, although the stray light is incident on the scanning imaging optical component, when the roughened surface is provided at the thus-incident place, the reflected beam is dispersed thereby, and is thus prevented from acting as ghost light. Accordingly, it is possible to effectively reduce or prevent generation of a ghost image.
A ghost image preventing method, according to the present invention, for preventing a reflected beam by a parallel plate from acting as ghost light in an optical scanning device, which deflects a beam from a light source by a light deflector having a deflection reflective surface rotating or swinging, condenses the deflected light toward a surface to be scanned by a scanning imaging optical system, forms a beam spot on the surface to be scanned and performs optical scanning of the surface to be scanned,
wherein:
a transparent parallel plate covering a window provided in a housing containing the light deflector for causing the beam from the light source to be incident on the deflection reflective surface, and, also, causing the beam reflected by the deflection reflective surface to exit from the housing is inclined with respect to a direction perpendicular to a deflection scanning plane; and
the scanning imaging optical system includes at least one rectangular lens,
wherein relative positional relationship between respective lenses and the parallel plate is set such that the reflected beam by the parallel plate is prevented from being incident on a side-end surface of the rectangular lens in a sub-scanning direction.
Thereby, generation a of ghost image is prevented by preventing the reflected beam by the transparent parallel plate from being incident on and reflected by a surface other than the effective optical surface of the scanning imaging optical component of the scanning imaging optical system.
For this purpose, the optical configuration may be set such that the following formula holds:
H1 less than 4(l+m1xe2x88x92xcex941)xcex8xe2x80x83xe2x80x83(1)
where:
xe2x80x98lxe2x80x99 denotes a distance between a reflection position xe2x80x98Axe2x80x99 of the deflected beam by the deflection reflective surface and the parallel plate;
xe2x80x98xcex8xe2x80x99 denotes an inclination angle of the parallel plate with respect to the direction perpendicular to the deflection scanning plane;
xe2x80x98m1xe2x80x99 denotes a distance between the lens surface nearest to the light deflector of the lenses included in the scanning imaging optical system and the reflection position A; and
it is defined that xe2x80x98xcex941=xe2x88x92H12/8R1xe2x80x99 by a radius R1 of curvature of the same lens surface; and a width H1 along the sub-scanning direction of the same lens surface.
The sign of the above-mentioned radius of curvature is such that the sign is plus when the lens surface is a convex surface viewed from the light deflector. The same manner will be applied hereinafter.
When the optical configuration is set as mentioned above, the reflected beam by the parallel plate is caused to stray in the sub-scanning direction, but is not incident on the optical component of the scanning imaging optical system. Accordingly, no reflection of the stray light by the optical component occur.
The scanning imaging optical system may consist of N ( greater than 1) rectangular lenses;
wherein an optical configuration is set such that l, mi, mi+1, Hi, Hi+1, xcex8, xcex94i and xcex94i+1 satisfy the following conditions:
Hi+1 less than 4(l+mi+1xe2x88x92xcex94i+1)xcex8xe2x80x83xe2x80x83(2) 
Hi greater than 4(l+mixe2x88x92xcex94i)xcex8xe2x80x83xe2x80x83(3) 
for any of I=1 through Nxe2x88x921,
where:
xe2x80x98Axe2x80x99 denotes a reflection position of the deflected beam by the deflection reflective surface;
xe2x80x98lxe2x80x99 denotes a distance between xe2x80x98Axe2x80x99 and the parallel plate;
xe2x80x98xcex8xe2x80x99 denotes an inclination angle of the parallel plate 4 with respect to the direction perpendicular to the deflection scanning plane;
xe2x80x98mixe2x80x99 denotes a distance between a lens surface 2I on a surface-to-be-scanned side of an I-th (1xe2x89xa6I less than N) lens from a light-deflector side of rectangular lenses of the scanning imaging optical system and xe2x80x98Axe2x80x99;
xe2x80x98mi+1xe2x80x99 denotes a distance between a lens surface 2I+1 on the light-deflector side of the (I+1)-th lens and xe2x80x98Axe2x80x99;
the following definition is made xe2x80x98xcex94i=xe2x88x92Hi2/8Rixe2x80x99
where:
xe2x80x98Rixe2x80x99 denotes a radius of curvature of the lens surface 2I; and
xe2x80x98Hixe2x80x99 denotes a width of the same lens surface along the sub-scanning direction; and
the following definition is made xe2x80x98xcex94i+1=xe2x88x92Hi+12/8Ri+1xe2x80x99
where:
xe2x80x98Ri+1xe2x80x99 denotes a radius of curvature of the lens surface 2I+1; and
xe2x80x98Hi+1xe2x80x99 denotes a width along the sub-scanning direction of the same lens surface.
When an adhered lens is included in the scanning imaging optical system, the adhered lens is counted as one lens.
Thereby, the stray light reflected by the parallel plate is not incident on any side-end surface of the lenses in the sub-scanning direction of the plurality of lenses included in the scanning imaging optical system. Accordingly, there is no possibility that the stray light is reflected by any side-end surface of lenses, and becomes ghost light.
An image forming apparatus, according to the present invention, of performing optical scanning of a photosensitive surface of a photosensitive medium by an optical scanning device, forming a latent image, and vis8alizing the latent image,
wherein the above-mentioned optical scanning device according to the present invention including the scanning imaging optical component according to the present invention is employed as the optical scanning device performing the optical scanning of the photosensitive surface of the photosensitive medium.
The photosensitive medium may comprise a photoconductive photosensitive body, the electrostatic latent image formed on the photosensitive surface through uniform charging and the optical scanning being visualized as a toner image.
In the above-mentioned image forming apparatus, the toner image may be fixed onto a sheet recording medium such as transfer paper, an OHP sheet (for an overhead projector), or the like.
As the photosensitive medium, a film for photography with silver halide may be used as the photosensitive medium, for example. In this case, the latent image formed through the optical scanning by the optical scanning device is visualized by a method of developing in an ordinary process of photography with silver halide. Such an image forming apparatus may be embodied as an optical plate-making system, or an optical drawing apparatus, for example.
The above-mentioned image forming apparatus according to the present invention may be applied to a laser printer, a laser plotter, a digital copier, a facsimile apparatus or the like.
Thus, according to the present invention, it is possible to render novel optical scanning device, scanning imaging optical system, scanning imaging optical component, optical scanning method, ghost image preventing method and image forming apparatus.
As the scanning imaging optical component according to the present invention has the roughened surface other than the effective optical surface, even a beam other than a proper scanning beam is incident on a surface other than the effective optical surface, it is dispersed, and, therefore, does not act as ghost light.
Further, in the ghost image preventing method according to the present invention, a beam other than a proper scanning beam is not incident on a side-end surface in a sub-scanning direction of a rectangular lens of the scanning imaging optical system. Accordingly, no generation of ghost light due to reflection by the side-end surface occur.
Accordingly, in the scanning optical system or optical scanning device employing this scanning imaging optical component, or in the scanning optical system or optical scanning device rendering the above-mentioned ghost image preventing method, generation of ghost image is effectively reduced or prevented, and, as a result, satisfactory image formation can be rendered.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.